Animal feed compositions can contain a variety of components, such as carbohydrates (sugars, starch products), fats (animal and/or vegetable fats), protein products, amino acids, vitamins and minerals. Various protein products are used in animal feed compositions, such as soya protein products, lactoproteins, maize gluten, wheat proteins, alpha-protein products, fish proteins and potato protein. Animal feed compositions are used as feed for calves, piglets, poultry, furred animals (minks, foxes), dogs and cats (pet foods) and fish (trout, carp, eel).
Potato protein is obtained as a by-product in the recovery of potato starch from potatoes. In the potato starch manufacture, using mechanical separation techniques, the potato is processed into potato starch, potato pulp and potato juice (also referred to as potato liquor). In the potato juice, the potato protein molecules are present in dissolved condition. There are various possibilities of isolating the potato protein from the potato juice in a more or less pure state. Usually, the potato juice is subjected to a heat treatment, as a result of which the potato protein molecules start to coagulate. This method is designated as heat coagulation or thermal coagulation. The thus-coagulated flocculent potato protein material can be separated from the liquid phase by means of filters, separators or decanters, yielding a separated wet potato protein product in the form of a wet cake. This product still contains 40-80% by weight of moisture and can subsequently be dried to 5-15% by weight of moisture. Calculated on a dry substance basis, heat-coagulated potato protein products contain about 70-90% by weight of protein (calculated as N.times.6.25), about 3-10% by weight of lipides, about 2-4% by weight of carbohydrates and 1-3% by weight of inorganic components.
Potato protein products have been used as a component in animal feeds over the past 50 years. For this application, potato protein has favourable properties in comparison with other vegetable protein products. The biological value of potato protein is particularly high because of the superior and balanced amino acid composition.
The separated wet heat-coagulated potato protein and the dried product obtained therefrom contain, in addition to the above-mentioned nutrients (proteins, fats, carbohydrates), contaminations in the form of sulphite, glyco-alkaloids, water-insoluble polymers of the melamine type and flavouring substances. In some cases, these contaminations can present problems in the application of animal feed compositions in which unpurified potato protein products are included as a component.
Glyco-alkaloids consist of carbohydrates which are glycosidically linked to a basic aglycone. In potato protein products, solanine and chaconine are the most important glyco-alkaloids. The total amount of glyco-alkaloids (TGA) in heat-coagulated unpurified potato protein products can vary between 500 and 5000 mg/kg (based on dry substance). It is known that glyco-alkaloids can give rise to poisoning symptoms upon consumption by humans or animals. Solanine possesses a direct toxicity due to its choline-esterase inhibiting action in the central nervous system. In addition, solanine has a bitter taste and gives a burning sensation upon consumption. If the glyco-alkaloid content in animal feeds is too high, undesired phenomena can occur, such as feed refusal and retardation of growth.
The water-insoluble polymers of the melamine type impart a grey to green colour to unpurified potato protein products. These polymers are formed by polymerisation (via a chinone stage) of phenol compounds (tyrosine, dihydroxyphenols such as in particular dihydroxyphenyl alanine, caffeic acid and chlorogenic acid) under the influence of enzymes present in the potato juice.
Undesired aromatic and flavouring substances in animal feeds have as a consequence that the animal feed is less well accepted by the animals in question.
If the above-mentioned contaminations are present in too high concentrations in animal feeds, problems can arise, such as a limited acceptance of the feed by the animals and retardation of growth. Owing to the presence of these contaminants, unpurified potato protein products in certain cases can be incorporated in animal feed compositions in limited amounts only. This applies in particular where young animals such as calves and piglets, and dogs are concerned.
The object of the invention is to provide a simple and inexpensive method for preparing purified heat-coagulated potato protein which is suitable for use in animal feed compositions.
Another object of the invention is to provide animal feed compositions in which purified heat-coagulated potato protein is present as a component. The above-mentioned problems that can arise in the application of unpurified potato protein are now solved entirely or largely. The animal feed compositions according to the invention can contain higher percentages of potato protein than if unpurified potato protein is used as a component.
In German Auslegeschrift 2,500,200 (column 8) the removal of solanine from heat-coagulated potato protein is described. The heat-coagulated potato protein (in the form of the wet flock or as a dried product) is treated (extracted) with an aqueous solution of acetic acid or citric acid or treated with an organic solvent (see Example (Beispiel) 3 and Tables (Tabelle) I and II). Through these treatments the solanine content of heat-coagulated potato protein is greatly reduced (see Tabelle I and II). It appears from Example 3a that the inorganic acid HCl gives much poorer results (no reduction of the solanine content) than acetic acid and citric acid when the treatment is carried out on coagulated potato protein which has not been separated from the potato juice in which the coagulation has occurred. The extracted potato protein products according to German Auslegeschrift 2,500,200 are applicable as a component in animal feeds and in foods for humans (see column 9).
The purification of separated heat-coagulated potato protein according to German Auslegeschrift 2,500,200 has as a drawback that the extracting agents used, acetic acid, citric acid or organic solvents are relatively expensive. The relatively high price of acetic acid and citric acid is also mentioned explicitly in German Auslegeschrift 2,500,200 (column 8, lines 1-12).